Tuesday, March 21, 2006

I keep thinking over how it might be possible to raise % efficiencies in polymer or nanocrystal liquid dispensed PV cells. I spoke with a close friend of mine, a Carleton University EE alumni, who is very skilled in semiconductor physics. I am of more modest skill in semiconductor physics (esp. at a quantum level) but often times I see what others seem to miss in process / device technology.

He brought up the salient point that any nanocrystal embedded in a weakly conducting (polymer) semiconductor will inherently be at a significant disadvantage as compared to more conventional monolithic grown or pulled semiconductor PV cells, largely due huge increases in parasitic cell resistances, - active material to semiconducting PV resistances on a nanoscale and poor bulk contact resistances to polymer conductors / semiconductors.

Same goes for a dense conglomerate of nanoparticles being of higher contact and bulk resistivity than a solid of similar materials.

My short conclusion in the interim is that several noted efforts seem to be technically weak mostly due to an "achilles heel" of parasitic resistances being excessively high.

His and my interpretation re desires to improve conversion efficiencies based on the nanoscale convertors alone, is that this possibly misplaced focus will miss the key point that the resistances are seemingly overwhelming as key issues, and not per se the convertors themselves ( at a nanoscale ).

While much protestation to the contrary is implicit due to the recent scientific coverage focusing on the nanoparticle convertor materials, by several well and even superbly credentialled folks, I have some doubts as to the merits being claimed.

It is clear that Quantum Dots ARE important, but the systems level cell materials issues ARE important - even equally and possibly more so. IE you can innovate in the Qdots and miss on the cell nano/microscale integration (ohmic losses in particular).

Cyrium's quick work with superb conversion results is a strong data point in support of my argument - they have done fantastic nanoparticle based PV conversion engineering and have by merits of the monolithic EPI growth, no high parasitic resistance issues common to Nanosys and Konarka - by Cyrium's best in in class materials choices...

And yes Cyrium's cells will not be able to be made roll to roll, nor on polymer, nor overall lowest in cell growth costs / area, in the foreseeable future, due to the issues related to EPI growth, but 39% conversion efficiency is nothing to sneeze at.

Can someone even come close in overall coversion efficiency by solution based nanoparticle / polymer PV convertors? Quite a stretch for now and possibly far into the future. Not impossible, but not likely yet.

Can Evident's wavelength convertors quantum efficiency, surmount the polymer / solution dispensed nanoparticle resistance issues? I have a degree of skepticism about that, until there are unusual huge breakthroughs in doping and conductances of polymer PV materials.

I may well be wrong, but I'd love to hear explanations why?

I also see one possible avenue to circumvent these challenges of parasitic resistance, but the process / materials issues implicit, are neither non-trivial nor without costs (ie how much closer can one get to Cryium's benchmark % eff, versus the cost increases in processing by some speculative means?)

The specific analogies one might infer, are that both Konarka and Nanosys's efforts which focus on the basic nanoscale particle convertor materials, in the hope that efficiencies will improve with careful but so far speculative experiment, might be missing the boat re the fact that fundamentally, conducting polymers (excluding metal particle doped "poly-mets" for obvious host matrix reasons) and especially semiconducting polymers, do not seem to be able to be produced with low resistivities in the range of the typical 0.5 ohm-cm seen in best in class silicon cells (nor approach the low contact resistivities of silicon or CS pv cells either).

Or do I have my figures wrong here - esp re semiconducting resistivities and with good low ?ms range carrier lifetimes?

This may well be a significant and possibly fundamental stumbling block to advances in solution processed nanocrystal type photovoltaic cells and polymer matrix hosted alternative photovoltaic convertors.

I'd hazard a guess that due to the properties of CIGS materials, this does not apply to NanoSolar's roll fed nanoparticle CIGS process. Despite the CIGS particle suspensions used as precursors, the material appears to have already high conversion efficiencies, and I'd tend to partially attribute this to CIGS low material resistivities versus the more avant gard polymer or semiconducting nanoparticle cell technologies.

1 Comments:

Anonymous said...

Mark,

You are on the right track, although the key parameter to focus on for organic semiconductors is the carrier mobility rather than the resistivity. (It turns out that it is very difficult to controllably dope these materials, so the general strategy is to use active layers that are nearly intrinsic, similar to the i-layer in an amorphous silicon cell.) Along with the carrier lifetime and absorption spectrum, the mobility plays an important role in determining the efficiency of a polymer solar cell.

I'd be glad to send you a review article on this stuff if you are interested.